1. Field of the Invention
The present invention relates to a transmission system having a transmitter coupled via a transmission channel to a receiver. The transmitter includes a channel encoder for encoding source symbols into coded symbols according to a coding property, and the receiver includes a source decoder for deriving reconstructed source symbols from the coded symbols received from the transmission channel. The transmission system further has a quality determining component for deriving a transmission quality measure, and a coding property setting component for setting the coding property to a value dependent on the transmission quality measure.
2. Description of the Related Art
Transmission systems can be used in applications where the quality of the transmission channel shows considerable variations. To enable a virtual error free transmission over such a transmission channel, the transmitter encodes the source symbols using a channel encoder according to an error correcting code having error correcting capabilities. In the receiver, the source symbols are reconstructed by a channel decoder. Useful codes can include convolutional codes and several types of block codes such as Reed-Solomon codes. Also a combination of a convolutional code with a block code is often used.
The ratio between the number of source symbols and the number of channel symbols of such a code is called the rate of the code. The error correction capabilities of such a code depend heavily on the rate of the code. In case of a transmission channel with a strongly varying transmission quality, the rate of the used channel code should be chosen to obtain virtually error free transmission at the worst channel conditions. This leads to a loss of useful transmission capacity when the transmission quality is high.
To prevent this loss of transmission capacity, the transmission system can set the at least one coding property (e.g., the rate of the channel encoder) in dependence on the transmission quality. If a Viterbi decoder is used as channel decoder, the transmission quality measure can be derived from the likelihood measures used in the Viterbi decoder.
Other quality measures can be, for example, constituted by a number of detected transmission errors per unit of time.
The problem to be solved by the present invention is how to derive a reliable quality measure despite of rapidly changing channel properties and changing coding properties.
To solve the problem with the prior art, a transmission system according to the present invention is characterized in that the transmission system comprises a filter for deriving a filtered transmission quality measure, and in that the transmission system comprises filter initializing means for setting the filter to a predetermined initial state at a changing coding property.
By using a filter to derive a filtered quality measure, an averaged quality measure is obtained which is suitable as basis for making decisions about the coding property. In order to reduce transient phenomena in the filter at the change of the coding property, the filter is set to a predetermined initial state. In one embodiment of the present invention, the filter is characterized in that the initial state corresponds to a typical quality measure for the changed coding property. By setting the initial state of the filter to a value corresponding to a typical quality measure, it is obtained that no transient phenomena occurs in the filter when the transmission quality corresponds to the typical value when the coding property is changed.
A further embodiment of the present invention is characterized in that a receiver comprises transmission means for transmitting the quality measure to the transmitter, in that a transmitter comprises a transmitter filter for obtaining a filtered quality measure, and in that a coding property setting means is arranged for setting the coding property in dependence on the filtered quality measure.
By transmitting the quality measure to the transmitter via a return link, the filter can be placed in the transmitter. It is observed that it is also possible that a filter is present in the receiver and the transmitter. The receiver is then arranged for transmitting via the return path a pre-filtered quality measure to the transmitter, which performs a further filter operation on the pre-filtered signal. The transmitter can be included in a base station for mobile communication, but it is also possible that the transmitter is included in the mobile station.